1. Field of the Invention
This invention relates in general to the field of precise position measurement in a three-dimensional workspace and more particularly to an improved apparatus and method of providing position-related information.
2. Description of Related Art
A variety of endeavors require or are greatly aided by the ability to make a precise determination of position within a three-dimensional workspace. For example, laying out a construction site according to a blueprint requires the identification at the actual construction site of a number of actual positions that correspond to features of the building on the blueprint.
Despite the many applications which require or are advanced by the ability to make precise determinations of position, it has historically been relatively difficult or expensive to precisely fix the position of any given point relative to an origin in an actual three-dimensional workspace.
A variety of techniques are known in the art to measure position, including land surveying techniques and global positioning satellite (xe2x80x9cGPSxe2x80x9d) system techniques. However, these techniques generally are not precise or require expensive devices which are complex and difficult to manufacture with high precision and accuracy. Additionally, many of these techniques required extensive training, and therefore cannot be practiced by those not trained in the art. Land surveying techniques, for example, fix position using a precision instrument known as a theodolite. The theodolite is both an expensive piece of equipment and requires substantial training to use. GPS equipment is relatively easy to use, but can be expensive and has limited accuracy on a small scale due to a certain amount of intentional error that is introduced by the military operators of GPS satellites.
Consequently, there has long been a need in the art for a method and device that can quickly and accurately fix positions in a three-dimensional workspace. There is a further need in the art for such a method and device which is easy to use and does not require extensive training.
It is an object of the present invention to provide a method and device that can quickly and accurately fix positions in a three-dimensional workspace. It is a further object of the present invention to provide such a method and device that is easy to operate and does not require extensive training.
Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims.
To achieve these stated and other objects, the present invention may be embodied and described as a position fixing system that includes, at a high level, several transmitters and a receiving instrument. The transmitters are preferably optical transmitters that transmit laser beams that have been fanned into a plane. The transmitters transmit signals from stationary locations and the receivers receive these signals. Consequently, the receiving instrument incorporates sensors, e.g., light detectors, that detect the signals from the transmitters. The receiving instrument then determines a coordinate system and calculates its position and assorted other information of interest from these received signals. The receiving instrument then displays this information through a user interface. The information may be, for example, the location of the receiving instrument or its distance relative to another location.
Various Figs. are included throughout this disclosure to illustrate a variety of concepts, components of several subsystems, manufacturing processes, and assembly of several subsystems.
1. Transmitter
The transmitter of the present invention includes a rotating head which sweeps one or more, preferably two, fanned laser beams continually through the three-dimensional workspace in which the receiver will be used to make position determinations based on the optical signals received from the transmitter. In this way, the signals from the transmitter cover the entire three-dimensional workspace. The present invention can be used in conjunction with the techniques and apparatus described in previous patent application U.S. Ser. No. 99/23615 to Pratt, also assigned to the present assignee, filed on Oct. 13, 1999, and incorporated herein by reference.
A. Simplified Optical Path
The receiver preferably has a clear optical path to each transmitter in the system during position fixing operation. One of the key advantages of the transmitters according to the present invention is the simplification of the optical paths as exemplified by the lasers rotating with the head. Additionally, there is no window in the preferred transmitter. Therefore, there is no distortion introduced by the movement of the laser beam across a window. As described in detail below, the preferred embodiment utilizes a lens or other device which rotates with the laser. Thus, there is no distortion caused, for example, by variable window characteristics or angles of incidence or between a rotating lens and a fixed laser. The absence of a fixed window also simplifies manufacture, maintenance, and operation. The absence of a fixed window does make it preferable that a rotating seal be added to the transmitter.
B. Speed of Rotation and Storage of Parameters
The rotating head of the transmitter of the present invention, and the lasers within it, rotate through a full 360 degrees at a constant, although configurable, velocity. As will be explained below, each transmitter in the system needs to rotate at a different velocity. Therefore, each transmitter has a velocity that can be controlled by the user. Additionally, each transmitter has an easily quantifiable center of rotation which simplifies the algorithms for determining position and can simplify the set-up of the system. A separate synchronization signal, also preferably an optical signal, fires in the preferred embodiment once per every other revolution of the rotating head to assist the receiver in using the information received from the transmitter.
The velocity of the rotating head is configurable through the use of, in the preferred embodiment, a field programmable gate array (xe2x80x9cFPGAxe2x80x9d). Such configurable speed control allows transmitters to be differentiated by a receiver based on their differing speeds of rotation. The use of multiple transmitters, as is appreciated by those of ordinary skill in the art, enhances position detection. Other advantages are obtained through the use of programmable electronics (FPGAs, flash memory, etc). Not only can the desired speed be set by changing the clock to the phase locked loop that controls the speed of rotation of the optical head, but the overall gain of the control loop can be programmed to maximize performance at the velocity of interest.
C. Beam Type and Number
As described in the incorporated provisional and known in the art, position detection is also enhanced by using multiple beams and controlling the shape of those beams. These beams may be in the same rotating head assembly or in separate rotating head assemblies.
Two beams is the preferred number per rotating head assembly, however, more beams can be used. In particular, another embodiment uses four beams, two for short range and two for long range. The two short-range beams have fan angles as large as possible. This allows the user to operate near the transmitters, such as in a room. For long-range, the user would normally be operating away from the transmitters. Therefore, in that circumstance the vertical extent of the beams is reduced to maximize the range of the system. The beams are, preferably, generated by Class III lasers. However, the rotation of the beams reduces their average intensity to the fixed observer such that the transmitters can be classified as Class I laser devices. Safety features are integrated into the device to prevent the powering of the lasers when the rotating head is not in motion. In the preferred embodiment at least two interlocks are utilized. The first depends on the phase lock loop that controls the rotational speed of the motor driving the optical head. The lasers are turned off until the system is rotating in phase-lock for at least 1024 phase-clock-cycles (approximately 32 revolutions). The second interlock monitors the absolute speed of the motor using the once-per-rev index on the encoder. A tolerance is programmed into the system, currently 1-part-in-1000. When the velocity is outside that window the laser is disabled and not allowed to operate.
D. Beam Shape
The Transmitter allows flexibility in setting beam characteristics as needed for the specific application of the invention. One advantage is that the beam shape can be modified. The key is that the beam shape should correspond with correctly filling the desired three-dimensional workspace. For construction trades this might be a room 20 mxc3x9720 mxc3x975 m in size. For construction machine control this might be a space 100 mxc3x97100 mxc3x9710 m in size. By modifying the beam shape, the optical energy can be properly directed.
The beam shape can also be controlled to differentiate beams. This can be done for multiple beams on a given transmitter or on different transmitters. For a given transmitter, the beam of the first and second beams must be differentiated. One technique uses their relative position with respect to the strobe in time. Another technique is to assure that the beams have different widths (xe2x80x9cbeam widthxe2x80x9d or xe2x80x9cdivergence anglexe2x80x9d). Then, for instance, the first beam could be the wider of the two beams.
Fanning the beam can be done using a variety of methods known in the art, including without limitation, rod lenses, pal lenses, and cylindrical lenses. The use of rod lenses offers a relatively simple approach, whereas the use of pal lenses offers greater control over the energy distribution. The beam typically is emitted from the source as a conical beam, then a collimating lens shapes the beam into a column, then the fanning lens fans the column.
Rod lenses can be used to increase control on divergence. One of the major advantages of rod lenses for line generation is that they do not directly affect the quality of the beam in the measurement direction (beam direction). Therefore, they should not affect the divergence of the laser beam as set by the collimating optics.
Pal lenses can be used to increase control of the energy distribution in the fan direction. PAL type lenses can even create xe2x80x9cuniformxe2x80x9d distributions, where the energy is uniform in the direction of the fan plane. A uniform distribution is often inefficient, however, if potential receivers are not uniformly distributed along the entire fan plane. In some implementations a focus must be created before the lens. In that implementation, the use of the PAL technique could affect the beam in the measurement direction.
Gaussian beams can also be used to maximize the performance of the receiver. Gaussian beams are symmetric beams in that the energy distribution across the divergence angle or beam width is symmetric. When a simple threshold technique is used in the receiver, it important that the pulses be symmetric and be without shoulders or sidelobes. It is also helpful if the distribution""s shape does not change with range. There are several pulse shapes that meet many of these criteria. However, the Gaussian distribution meets all of these criteria. With symmetric pulses that do not have shoulders or sidelobes, the receiver will be able to detect the center of the beam. Non-symmetric pulses, conversely, can cause the receiver to falsely identify the exact time when the beam center intersects the receiver""s optical detector.
E. Strobe
Additionally, the synchronization signal, mentioned above, is strobed and must be symmetric. Therefore, pulse shaping in the flash/strobe pulse generator for the synchronization signal is required. A square pulse with equal rise and fall times is one desired pulse shape. A Guassian shaped pulse similar in shape and duration to the pulse created by the scanning laser beams in an optical detector is another desirable shape. This pulse is preferably provided to a plurality of LEDs on the transmitter that are arranged to send out the synchronization pulse signal in a multitude of directions throughout the three-dimensional workspace. The light output of the LEDs is directly proportional to the current flowing through the LEDs. Because of the high currents involved in creating the strobe, a pulse-forming network must be used to assure that the current is a square pulse, or other symmetric pulse, as it passes through the diodes.
F. Communications and Control
A transmitter according to the present invention uses a serial port for communication and control. This allows calibration data and control parameters to easily be transferred. Recall that the transmitters are differentiated by their speeds. Therefore a technique must be put in place to simplify speed changes. Additionally, a particular set of transmitter parameters must be made available to the receiver so that the receiver can calculate position based on the signals received from the transmitter. To create a simple, reliable, and unified technique the preferred embodiment uses serial communication between the transmitter and the receiver or test equipment. For test purposes, the serial connection is a well-known RS-232 connection. For use in the field, the connection is preferably through an infrared serial port. This allows the transmitter to be sealed and yet communicate with the outside world. To avoid interference with the measurement technique, this port is only active when the lasers are off.
G. VHDL
Many of the digital designs of a disclosed embodiment are implemented in field programmable gate arrays (FPGAs). These devices allow complex designs to be programmed into general-purpose hardware available from multiple vendors. The programs for these devices are written in a special computer understandable language VHDL (VHSIC [very high-speed integrated circuit] Hardware Description Language). This is the same language that is used to design microprocessors and other semiconductor devices and is now standardized as IEEE 1076.
H. Providing Power to the Laser Head
As explained in the incorporated provisional application, the motor and the provision of power to the rotating head assembly are key components of a transmitter according to the preferred embodiment. For accuracy in position fixing with the applicants"" apparatus, it is imperative that the laser beams rotate with a precisely constant angular velocity. Any mechanical or electrical component that may cause angular velocity to vary must be avoided.
A rotary transformer is used to transfer electrical power to the rotating head of the transmitter. Several techniques are available for powering devices in a rotating head. The most common is the use of slip rings. Unfortunately, slip rings require physical contact between the brushes and the slip-ring. This creates dust in the system and can cause time varying friction on the motor shaft. The preferred technique is to use a rotating transformer. The transformer technique will provide minimal drag on the motor that does not vary through the rotaton of the optical head and which therefore will not cause changes in the angular velocity of the head during one revolution. Additionally, through the use of flat signal transformers as power transformers, the technique is very compact.
Fly-back control is used on the stator side of the transformer. To minimize the number of components in the rotating head, the voltage control is performed on the stator side of the transform. To optimize efficiency, a fly-back driving technique is utilized.
I. Stability and Precision of Rotation
The stability of the rotational velocity control system and drive motor is also discussed in the incorporated provisional application. As those of ordinary skill in the art will recognize, a brushless sine wave drive motor is a low-cost motor with good inherent stability intra-revolution and, as such, is useful in ensuring constant velocity rotation. Inertia of the rotating head is another important factor in achieving the required stability of angular velocity.
The bearing separation should be maximized to achieve optimal results. Any precession and wobble (wow and flutter in a turntable) will be a source of error in the system. It will lead directly to an error in the xe2x80x9czxe2x80x9d direction. Using two precision bearings and maximizing the distance between the bearings can minimize these errors.
The strobe pulses of the synchronization signal are based on a once-per-revolution indicator tied to the motor shaft. There are many ways to create this shaft position index. The simplest and preferred technique is to use the index normally supplied with an optical encoder. This separate output of the encoder is directly equivalent to a shaft position index. An optical encoder disk is used to give rotation information. Other devices, including without limitation, tachometers and synchros could be used.
The optical encoder disk is typically made of glass and has a series of radial marks on it which are detected as the disk rotates. Additionally, the disk typically has a single xe2x80x9cindexxe2x80x9d mark of a different radius which is used to detect complete rotations. The disk system produces a square wave with a frequency dictated by the speed with which the radial marks are passing. For example, if the disk is rotating at 1 revolution/second, a 1000 mark disk system would produce a 1000 Hz square wave (1000 radial marks/revolution*1 revolution/second=1000 Hz).
The speed of the motor is controlled through a feedback phase-locked loop (xe2x80x9cPLLxe2x80x9d) system. The disk system square wave is one input and a clock from the transmitter system is the other input. The transmitter clock has a selectable frequency. The output of the PLL is used to control the speed of the motor rotation such that the PLL remains locked at the selected frequency.
The index mark of the disk can also be used to initiate the strobe pulse as often as once per revolution.
J. Low Manufacture Cost
As noted above, for the receiver to use the signals from the transmitter to accurately and precisely fix a position in the three-dimensional workspace, the receiver must have available a certain set of parameter characteristic of the transmitter. For example, as will be explained in detail below, the receiver must know the angles at which the laser beams are emitted from the transmitter head.
If these angles are pre-defined and the transmitter must then be manufactured with precision to match the specified parameters, it becomes extremely expensive to manufacture the transmitter. Under the principles of the present invention, the transmitter can be manufactured without such high precision and without requiring that the resulting transmitter conform to pre-specified parameters. Rather, the operating parameters of the transmitter required by the receiver are carefully measured after the transmitter is manufactured. This process, which will be referred to as transmitter calibration herein and described in more detail below, removes the expensive requirement of a precisely constructed transmitter. Consequently, the system of the present invention becomes much less expensive.
The operating parameters of the transmitter are preferably stored electronically in the each transmitter after that transmitter is calibrated. Consequently, each transmitter preferably incorporates a memory device in which the calibration parameters can be stored. These parameters can then be communicated to the receiver electronically through the serial port, optical or wired, of the transmitter described above. The receiver will have a corresponding serial port, optical or wired, for receiving data from the transmitter.
Preferably, these parameters are then stored in memory in a Position Calculation Engine (PCE) of the receiver and can be updated as required. For example, if a new transmitter is added to the system, then a new set of parameters needs to be loaded into the PCE from or for that transmitter. As an additional example, if the rotation speed of a transmitter is changed, then this information needs to be updated in the PCE.
2. Receiving Instrument
In the present system, the receiver or receiving instrument is a wand, an example of which is illustrated in FIG. 17. The wand provides a light-weight, mobile receiving instrument that can be carried anywhere within the three-dimensional workspace. A tip of the wand is used as the point for which position within the workspace is determined based on the signals received from the system transmitter. The position of the tip can be continuously calculated by the receiver and displayed for the user on a display device provided on the receiver. Consequently, no extensive training is required to operate the position fixing system of the present invention once the system is set up and functioning.
The wand preferably contains two receivers, which are light detectors if the transmitter is emitting optical signals as in the preferred embodiment. The Position Calculation Engine (xe2x80x9cPCExe2x80x9d) of the receiving instrument is a processor that performs most of the computations of the receiving instrument. The PCE supports any required set-up procedure as well as the subsequent tracking, position calculation, and information display functions. The receiving instrument and PCE will be described in detail below.
The Smart Tip, shown in FIG. 17, can also perform computations, as indicated by the FPGA (field-programmable gate array) and the xe2x80x9ci Buttonxe2x80x9d in each Smart Tip. The Smart Tip can be present at either end of the wand in the present system and the signal xe2x80x9cTip Presentxe2x80x9d indicates whether there is a Smart Tip on each of the receiving instrument ends.
3. Setup
The setup procedure is described in detail below. The setup procedure places the transmitters in position and commences their operation. The setup procedure also allows the system to, among other things, define a useful coordinate system relative to the three-dimensional workspace and begin tracking the wand""s location in that coordinate system.